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Rev. 04
STP 3 & 4
Final Safety Analysis Report
19.4 External Event Analysis and Shutdown Risk Analysis
The information in this section of the reference ABWR DCD, including all subsections,
is incorporated by reference with the following departures and supplements.
STD DEP T1 2.15-1
STP DEP 19R-1
STD DEP Admin
19.4.3.2.1 Structural Fragility
STD DEP T1 2.15-1
The radwaste building does not contain safety-related equipment and its failure will not
lead to core damage. Consequently, an estimate of the radwaste buiilding fragility is
not required.
19.4.3.4 Results of the Analysis
The following site-specific supplement addresses additional results of the analysis.
The STP 3 & 4 site-specific geology is bounded by the reference ABWR DCD seismic
design.
19.4.4 Fire Protection Probabilistic Risk Assessment
The following site-specific supplement addresses additional results of the analysis.
The ABWR FIVE analysis was reviewed as discussed in Appendix 19M, based on the
proposed plant departures and STP 3 & 4 site-specific characteristics. The existing
ABWR FIVE results are considered bounding for the STP ABWR.
19.4.5 ABWR Probabilistic Flooding Analysis
The ABWR probabilistic flooding assessment considered internal flooding and external
flooding events. The results of the internal and external flooding assessments are
discussed below.
STD DEP Admin
STP DEP 19R-1
The results of the ABWR Probabilistic Internal Flooding Analysis show that the turbine,
control, and reactor buildings are the only structures that required evaluations for
potential flooding. The other buildings do not contain any equipment that could be used
for safe shutdown or potential flooding would not result in a plant transient.
The following site-specific supplement addresses probabilistic flooding analysis of the
relocated Reactor Service Water pump house.
External Event Analysis and Shutdown Risk Analysis
19.4-1
Rev. 04
STP 3 & 4
Final Safety Analysis Report
The results of the ABWR Probabilistic Internal Flooding Analysis show that the turbine,
control, and reactor buildings and the Reactor Service Water pump house, are the only
structures that required evaluations for potential flooding. The other buildings do not
contain any equipment that could be used for safe shutdown or potential flooding would
not result in a plant transient.
Flooding in the turbine building could result in a turbine trip due to loss of circulating
water or feedwater. Automatic pump trips and valve closure on high water level should
terminate the flooding. But if these were to fail, a non-watertight door at grade level in
the turbine building should allow water to exit the building. If this door retained water,
watertight doors would prevent water entering the control and reactor buildings. The
core damage frequency (CDF) for turbine building flooding is extremely small.
The worst case flood in the control building is a break in the reactor service water
system (RSW) which is an unlimited source. Floor drains and other openings in the
floor would direct all flood water to the first floor where the reactor building cooling
water (RCW) rooms are located. The RCW rooms contain sump pumps. Water level
sensors in the RCW rooms should actuate alarms in the control room and send signals
to trip the RSW pumps and close isolation valves in the RSW system. If these sensors
were to fail, watertight doors on each room should limit flood damage to only one of the
three RCW divisions. Breaks in the fire water system could result in interdivisional
flooding in the upper floors but floor drains would limit water height to below installed
equipment for the first hour. To prevent damage to safety-related equipment after this
time requires operator actions to limit the depth of water. The CDF for control building
flooding is extremely small.
The RSW pump house is contiguous with the Ultimate Heat Sink (UHS), and is
separated into three divisions each with two levels, the pump room at elevation (-)18
feet nominal, and the Electrical and HVAC room at elevation14 feet nominal. Each
division is separated from the other divisions by watertight, three-hour fire rated doors
at both elevations (RSW Interface Requirements, Subsection 2.11.9 (2)). The normal
operating level in the UHS basin is 63 feet 3 inches to 71 feet, or approximately 37 feet
above site grade (nominally 34 feet). For each RSW division, the RSW supply line from
the Ultimate Heat Sink (UHS) splits in the RSW pump house to provide water to both
RSW pumps. The RSW pump discharge combines into a single supply line to the
Control Building. RSW return from the Control Building passes through the RSW pump
rooms and returns to the UHS above the normal operating level. Flooding in the RSW
pump house could occur from failure of the supply line from the UHS, or from failure in
the RSW return line to the UHS. The RSW pump rooms contain sump pumps and
water level sensors that function to mitigate the effects of small breaks or leaks from
the RSW lines in the RSW pump room. If the sensors were to fail, watertight doors on
each room and level should limit flood damage to only one of the three RSW divisions.
Large breaks in the supply lines from the UHS are unisolable before the pump
discharge isolation motor-operated valve (MOV). Breaks after the pump discharge
MOV and in the RSW return line to the UHS are isolable with the pump discharge MOV,
which closes automatically on high level in the RSW pump room. Unisolable breaks in
the RSW supply piping will result in draining the UHS to El. 50’ through the ventilation
intake ducts in the top of the RSW pump house. Breaks in the fire water system could
19.4-2
External Event Analysis and Shutdown Risk Analysis
Rev. 04
STP 3 & 4
Final Safety Analysis Report
result in interdivisional flooding in the upper floors of the RSW pump house, but floor
drains would limit water height to below installed equipment for the first hour. To
prevent damage to safety-related equipment after this time requires operator actions
to limit the depth of water. The CDF for internal flooding in the RSW pump house is
very small.
The following site-specific supplement addresses probabilistic flooding analysis for
external flooding.
The ABWR Probabilistic External Flooding Analysis screened all but one external
flooding event from consideration because flood waters would not rise to an elevation
above the entrances to plant buildings. The external flooding event with the potential
to result in core damage is a breach of the main cooling reservoir.
The external flooding event is assumed to cause a non-recoverable loss of offsite
power as well as fail all equipment in the turbine building and the fire protection pump
house.
Failure of any watertight door to prevent external flood waters from entering the reactor
building was assumed to result in core damage since the essential AC switchgear is
located below grade and there are no internal watertight barriers that would prevent
water that enters the reactor building from causing failure of all three AC divisions.
Failure of any watertight door to prevent water from entering the control building was
assumed to result in core damage because all three essential DC divisions and the
main control room are located below grade and there are no internal watertight barriers
that would prevent water that enters the control building from failing all three DC
divisions or the main control room. For a breach of the main cooling reservoir, timely
operator action is required to close the normally-open main control room access door.
Additionally, failures that resulted in a station blackout were assumed to be nonrecoverable and result in core damage.
The total CDF from external flooding events is very small.
External Event Analysis and Shutdown Risk Analysis
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